Disabling Shared Video Memory?

Does anyone know if it's possible to disable shared video memory? For some reason shared memory is being added to the available pool of VRAM of which there is 512MB already so system RAM needn't be used AFAIK. I'm not sure if this would impact performance at all but as I said above is there a way to disable this 'shared system memory'?
Thanks
http://h.imagehost.org/0667/VRAM.png

Don't believe you can as its a windows vista and upwards performance "feature", could be wrong though.